Thin film batteries (TFBs) have been projected to dominate the micro-energy applications space. TFBs are known to exhibit several advantages over conventional battery technology such as superior form factors, cycle life, power capability and safety. FIG. 1 shows a cross-sectional representation of a typical thin film battery (TFB) and FIG. 2 shows a flow diagram for TFB fabrication along with corresponding plan views of the patterned TFB layers. FIG. 1 shows a typical horizontal TFB device structure 100 with anode current collector 103 and cathode current collector 102 are formed on a substrate 101, followed by cathode 104, electrolyte 105 and anode 106; although the device may be fabricated with the cathode, electrolyte and anode in reverse order. Furthermore, the cathode current collector (CCC) and anode current collector (ACC) may be deposited separately. For example, the CCC may be deposited before the cathode and the ACC may be deposited after the electrolyte. The device may be covered by an encapsulation layer 107 to protect the environmentally sensitive layers from oxidizing agents. See, for example, N. J. Dudney, Materials Science and Engineering B 1 16, (2005) 245-249. Note that the component layers are not drawn to scale in the TFB device shown in FIG. 1.
However, there are challenges that still need to be overcome to allow cost effect high volume manufacturing (HVM) of TFBs. Most critically, an alternative is needed to the current state-of-the-art TFB device patterning technology used during physical vapor deposition (PVD) of the device layers, namely shadow masks. There is significant complexity and cost associated with using shadow mask processes in HVM: (1) a significant capital investment is required in equipment for managing, precision aligning and cleaning the masks, especially for large area substrates; (2) there is poor utilization of substrate area due to having to accommodate deposition under shadow mask edges and limited alignment accuracy; and (3) there are constraints on the PVD processes—low power and temperature—in order to avoid thermal expansion induced alignment issues.
In HVM processes, the use of shadow masks (ubiquitous for traditional and current state-of-the-art TFB fabrication technologies) will contribute to higher complexity and higher cost in manufacturing. The complexity and cost result from the required fabrication of highly accurate masks and (automated) management systems for mask alignment and regeneration. Such cost and complexity can be inferred from well-known photolithography processes used in the silicon-based integrated circuit industry. In addition, the cost results from the need for maintaining the masks as well as from throughput limitations by the added alignment steps. The adaptation becomes increasingly more difficult and costly as the manufacturing is scaled to larger area substrates for improved throughput and economies of scale (i.e., HVM). Moreover, the scaling (to larger substrates) itself can be limited because of the limited availability and capability of shadow masks.
Another impact of the use of shadow masking is the reduced utilization of a given substrate area, leading to non-optimal battery densities (charge, energy and power). This is because shadow masks cannot completely limit the sputtered species from depositing underneath the masks, which in turn leads to some minimum non-overlap requirement between consecutive layers in order to maintain electrical isolation between key layers. In addition, there are the inherent alignment limitations that come from inaccuracies in mask fabrication and alignment accuracies of an alignment tool. This alignment inaccuracy is further exacerbated by the thermal expansion mismatch between the substrate and the mask, as well as the differential thermal conditions during the deposition. The consequence of this minimum non-overlap requirement is the loss of cathode area, leading to overall loss of capacity, energy and power content of the TFB (when everything else is the same).
A further impact of shadow masks is limited process throughput due to having to avoid additional thermally induced alignment problems—thermal expansion of the masks leads to mask warping and shifting of mask edges away from their aligned positions relative to the substrate. Thus the PVD throughput is lower than desired due to operating the deposition tools at low deposition rates to avoid heating the masks beyond the process tolerances.
Furthermore, processes that employ physical (shadow) masks typically suffer from particulate contamination, which ultimately impacts the yield.
Therefore, there remains a need for concepts and methods that can significantly reduce the cost of HVM of TFBs by enabling simplified, more HVM-compatible TFB process technologies.